One of the most important resources for human support is potable water. Large populations in our world lack access to potable water and access to adequate sanitation. In addition, potable water is important for long-term human missions in space, where such water may be vital for consumption, hygiene, and maintenance. Since supplies of potable water may not be readily available, water reclamation to generate potable water from wastewater is essential. Sources of wastewater in long-term space missions can consist of hygiene water, laundry water, humidity condensate, brines, and human waste (e.g., urine). Due to the high cost of delivering supplies to space, recovery of potable water from wastewater may be critical to life support of crew members. Long duration space missions to the moon, Mars, and near-Earth asteroids may be mass-constrained and may require robust and reliable life support hardware. Closing the water loop on long duration space missions can be crucial to reducing mission mass, cost, and logistics support for orbiting facilities and planetary spacecraft.
Water recovery from wastewater is not only important in space applications, but can also be important in terrestrial applications. Such terrestrial applications of water recovery can include water recycling in arid regions, water treatment for disaster relief, greywater recycling onboard ships, and water recycling at long-term military outposts, ships, and submarines.
One type of wastewater for closing the water loop can include urine. On the International Space Station (ISS), urine may be stabilized using pretreatment chemicals, such as chromium trioxide and sulfuric acid, at a waste collection system. Typically, water can be recovered from the pretreated urine using a Vapor Compression Distillation (VCD) system. The VCD system is capable of recovering about 75% of water from the pretreated urine. However, the VCD system is very complex and uses several moving parts. Furthermore, the VCD system produces brine that requires further processing for water recovery.
Due to the VCD system's complexity and reduced capacity, membrane technology has been developed to simplify water purification systems. One such membrane-based strategy is reverse osmosis membrane technology and another such membrane-based strategy is forward osmosis membrane technology. While both reverse osmosis membranes and forward osmosis membranes may be effective in limiting surfactants, both are unable to reject urea, which is a small, uncharged contaminant molecule typically found in urine. As a result, such membranes may be supplemented with a second process capable of filtering out urea. Osmotic distillation and membrane distillation technology may be used to reject urea, but are not effective in limiting low-surface tension fluids, such as surfactants. When integrated together, this leads to the complexity and costs of water recovery from wastewater. Also, having to use different systems to treat different streams of wastewater can present problems from a mass, power, cost, logistics, and volume perspective. A single practical process that is capable of extracting purified water from urine in a single step may be beneficial in closing the water loop.